1. Field of the Invention
The invention relates, in general, to electroluminescent screens of the type that comprises a thin electroluminescent layer based on compounds of elements of zinc and sulphur, on a transparent support. More precisely, it relates to a process of manufacture of the electroluminescent layer, appliances for application of the process and the electroluminescent layers provided by the same process. Screens of this type have a sandwich structure and comprise, as an example:
a support plate of transparent material (glass); PA1 a first electrode composed of a continuous layer of transparent and electrically conductive material (SnO.sub.2 or In.sub.2 O.sub.3 or a mixture of the two) deposited on the support; PA1 an electroluminescent layer deposited on the first electrode; PA1 a second electrode or counter-electrode composed of a continuous layer or of strips of electrically conductive material (aluminium) deposited on the electroluminescent layer. PA1 (a) the luminance emission power defined by the ratio of the luminance to the excitation voltage; PA1 (b) the ratio of the luminance to the width of pulses in the event of pulse excitation; PA1 (c) the discrimination ratio defined as the ratio between luminance obtained by application of a voltage U, continuous or pulsed, and the luminance obtained by application of a voltage U/2; PA1 (d) the lifetime which is defined as the time of operation under given conditions of excitation, at the end of which luminance only attains half its initial value. PA1 excitation is difficult with DC current supply; PA1 sensitivity is weak, in other words luminance has to be excited by a voltage of value of at least some tens of volts and this significantly reduces the possibilities of providing power to the screen through common types of semiconductor devices. PA1 in order to favour adherence of the electroluminescent layer on the substrate and texturization of the layer, the support is fitted with facilities for heating the said substrate that enable it to be held at a suitable temperature while the deposit is being applied and then, at the end of depositing process, for heating the substrate in order to homogenize by recrystallizing the microstructure of the layer; PA1 in order to provide an additional effect of cryopumping, the ovens are surrounded by a double vapor-trap wall through which a cooling fluid passes; PA1 in order to reduce heat radiation between ovens, each oven is surrounded by a heat screen; PA1 in order to delineate the geometry of the vapor stream of each component, each oven is surrounded by a diaphragm containing a central opening which provides the passage for the vapor and is arranged and orientated so that the axes of the vapor streams converge towards the substrate.
The material or phosphor of which the electroluminescent layer is made is composed of zinc and sulphur. Zinc sulphide (ZnS) is the most commonly used. In order to obtain an acceptable rate of emission brightness, at least one doping element selected from copper metal and manganese metal must be incorporated in the form of an activator to the electroluminescent compound. Selection of doping elements provides a means of acting to a certain extent on the spectral composition of a light beam excited by application across the electrodes of an alternating or direct voltage.
Further, the functional properties depend to a very great extent on the structure of the electroluminescent layer which is itself determined by the preparation process.
The following parameters are particularly used to assess these processes:
Depending on the process of manufacture utilized, the phosphor electroluminescent layer may be made of phosphor grains dispersed in a transparent dielectric binding agent or may take the form of thin evaporated films. These latter may be excited by a high alternating voltage and this leads to good lifetimes or by a direct or pulsed voltage but with poor lifetimes.
2. Prior art
A known process of manufacturing a granular layer consists of spreading a suspension of a powder of a doped compound, agglomerated by a small quantity of dielectric binding agent, on a substrate which has previously been covered by a light transmissive, electrically conducting electrode. The binding agent may advantageously be a polymer resin. It has been strongly recommended, in order to obtain a very fine powder of homogeneous composition, that it should be prepared by co-precipitation from a solution containing the electroluminescent compound (ZnS) and the doping elements (Cu and Mn). In order to provide direct current operated electroluminescent devices, the surface of the grains is covered by diffusion with copper sulphide; it is also necessary, as in the other processes, to carry out an operation of forming of the layer by application of an unidirectional voltage for a certain time. The DC current supply required to obtain a suitable luminance after forming, that is to say better than ten foot lambert, is about 100 volts. Lifetimes of the order of 2000 hours can be reached with these devices.
A process of high-frequency sputtering has also been proposed to provide a continuous layer. This process consists of placing the substrate on an electrode in an evacuated bell jar in the vicinity of further target electrodes made of the bodies constituting the electroluminescent layer to be formed and applying a high frequency sufficiently high voltage between the substrate electrode and the target electrodes to generate plasmas. The electroluminescent layer obtained is fragile and it has been established that a resistant deposit of cermet (nickel-silica or aluminium-silica) must be applied to the said layer to limit the operating current and to thus increase the breakdown voltage. In any case, the lifetime of such layers, excited by pulses of medium voltage (greater than 250 V), would not appear to exceed a few hours.
According to another process of the prior art, a method of manufacturing a thin film electroluminescent device, wherein the said thin film of electroluminescent material is composed of a matrix material consisting of one or more of the compounds zinc or cadmium sulphide or selenide, activated by at least two activator metals and at least one halogen so as to be excitable to luminescence by the application of a voltage between electrodes, includes the steps of first evaporating the matrix material simultaneously with at least one of the required activator metals in free or combined form and causing the evaporated substances to be deposited together, in the desired relative proportions and in the form of a thin film on a light-transmissive, electrically conducting substrate constituting a support for the said film and one of the electrodes of the device, then raising the temperature of the substrate and film and exposing the film to a gaseous mixture consisting of or containing the remainder of the required activator elements, including one or more halogens, in the vapor state, the temperature of the substrate, the vapor pressure of the said gaseous mixture, and the time of exposure of the film to said mixture being such as to cause the desired amounts of said elements to be deposited upon and to diffuse into said film, and recrystallisation of the film to take place, and then depositing a metal layer upon the film by evaporation to form the second electrode of the device.
The electroluminescent layers of the prior art have at least one of the following disadvantages:
The main reason for these disadvantages seems to be the lack of homogenity of their composition and of their internal structure. Thus, for instance, when the layer is built up by vaporization in vacuum of a powder containing one of the basic compounds, the high temperature required which is at least 1,200.degree. C. in the case of zinc sulphide and more if it is desired to hasten the rate of deposit, is likely to cause partial decomposition of the said compound; the result is that the composition of the layer obtained is not stoechiometric. Further, when the matrix material contains basic compounds and activator metals, it is virtually impossible to give the said matrix a composition which provides optimization of both the rate of deposit of the basic compounds and the rate of deposit of the activator metals.
The process of the invention is simple, only requires equipment of relatively low cost and provides electroluminescent thin film devices which have satisfactory lifetimes and high sensitivity, whether operated by DC current supply or by pulsed current over a wide range of pulse widths. Further, screens may be made with completely reproduceable characteristics and with a large surface area, viz 100 cm.sup.2.